1. Field of the Invention
This invention relates to an optical apparatus for tracking a record carrier and reading out prerecorded information therefrom, such as video and/or audio signals, and more particularly to an optical apparatus comprising an improved means of tracking control.
2. Description of the Prior Art
Optical video disc players are well known in the prior art, as for example the Philips-MCA system, and Pulse Code Modulation (PCM) systems in which audio signals are reproduced are also well known.
The disc player is generally provided with a radiation source such as a semiconductor laser for projecting a light beam onto a record carrier, such as a record disc, an object lens for focusing the light beam on the disc, drive means for causing relative motion between the light beam and the disc in order to optically scan the surface of the disc and read out means for reproducing the information signal in accordance with the intensity of the beam reflected from the disc.
More specifically, the disc player is typically constructed in such a way that the record disc is spirally scanned with the light beam slowly moving along a radius of the rotating disc to read out the information signal in accordance with the intensity of the reflected beam, which varies with the presence of information pits.
Generally speaking, the record discs on which the tracks are encoded spirally or concentrically are necessarily subjected to the effects of eccentricity or an out-of-roundness of the tracks induced by mechanical inaccuracy. In order to correctly follow these minute tracks (hereinafter referred to as "tracking") at the time of reproduction, it is known to provide two light spots, one for reading out the signals, and the other for tracking. To facilitate an understanding of this invention, a detailed description of the prior art is presented below.
A conventional optical reproducing system is shown in FIGS. 1 through 3. In FIG. 1, a radiation source 1 consists of two semiconductor lasers 1a, 1b whose radiation sources are in the same plane. The laser beams 2,2 radiated from the source 1 are respectively collimated by a first lens 3. The collimated beams pass through a half mirror 4, and are focused by a second lens 5 onto the surface of a disc 6 on which a track 7 is formed.
As shown in FIG. 2, the beams focused on the track 7 form two spots 8a, 8b; the spot 8a is used for the readout of signals, and the spot 8b for the tracking of the track 7. The laser beams 2,2 reflected by the track 7 are again collimated by the lens 5. The collimated beams are reflected by the half mirror 4 and are again focused via a third lens 9 onto radiation sensitive detectors 10a, 10b.
More specifically, the reflected beam from the spot 8a is fed to the detector 10a which converts the optical signals, which vary in intensity in accordance with the presence of the pits on the track 7, into electric signals. The output from the detector 10a is fed to a demodulating circuit (not shown) via a preamplifier 11a. The reflected beam from the spot 8b is fed to a detector 10b that is connected to a tracking control circuit.
FIG. 2 (a) shows the relative position of the spots 8a and 8b on the track 7. The diameter of each spot is about 2 .mu.m, and the distance between them is several tens of .mu.m, although such spacing is shortened in the drawing for simplicity of illustration. The relative position of the spots 8a, 8b is always maintained constant.
FIG. 2 (b) shows a characteristic curve representing the variation of the reproduced signal level corresponding to the relative positional deviation between the center of the track and the center of the spot. The x axis represents the amount of deviation of the spot center with respect to the track center, and the y axis represents the output level of the reproduced signal. Accordingly, when the spot 8b is positioned at either of points B' on the curve, the output level of the detector 10b is represented by B. At this time, because the spot 8a is positioned at the center of the track, the output level of the detector 10a reaches the maximum point A.
If the position of spot 8a shifts in either the plus direction or the minus direction, the output level of the detector 10a will decrease in accordance with the curve illustrated in FIG. 2 (b). However, because the position of spot 8b is also moved in accordance with the shifting of spot 8a, the output of detector 10b will change in inverse proportion to the polarity of the shifting direction of spot 8b as seen from the Figure, the changing rate of the output level being determined by the inclination of the curve at point B'.
Referring back to FIG. 1, the output of the detector 10b, containing the track shifting information due to the deviation from the reference position B' of spot 8b, is fed to a spot shifting device 14 by way of a preamplifier 11b, a comparator 12 and a servo-amplifier 13. As a result, the spot shifting device 14 is operable to move spots 8a, 8b back into the predetermined position, thus following the relative lateral shifts of the track.
FIG. 3 shows an embodiment of the spot shifting device 14, which is similar to the voice coil of a loudspeaker in its construction. The device 14 comprises a permanent magnet pole 15, a core 16, a coil 17, springs 18, a bobbin 19a and a connecting bar 19b. The coil 17 is wound on the exterior of the bobbin 19a, which is supported by springs 18, 18 whose ends are connected to a fixed support. In accordance with the magnitude of the current flowing in the coil 17, the magnitude and direction of movement of the coil 17 is determined. Thus, the connecting bar 19b is moved as shown by the arrow. The connecting bar is fixed to the optical system, and thus functions to move the spots 8a, 8b back into their optimum predetermined positions.
This prior art tracking control device has a disadvantage in that the structure and control of the optical system becomes complicated since an additional lens 9 and two radiation-sensitive detectors 10a, 10b are needed in order to separate the reflected beams. Consequently, it is very difficult to accurately focus the reflected beams on the radiation-sensitive detectors 10a, 10b since the diameter of the focused spots is only several .mu.m and the distance between them is several tens of .mu.m.